Mixer

ABSTRACT

A mixer for mixing first and second fluids has a passageway along which a stream comprising the first fluid flows along an axis of the passageway. The passageway has, in sequence in the downstream direction, a convergent section, a throat, and a divergent section. An injector introduces the second fluid into the stream in the passageway upstream of the divergent section. A swirl generator in the passageway is upstream of the convergent section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. National Stagedesignation of co-pending International Patent ApplicationPCT/EP2004/050074 filed Feb. 3, 2004, which claims priority to GreatBritain patent application no. 0303495.6 filed Feb. 14, 2003, and theentire contents of these applications are expressly incorporated hereinby reference thereto.

FIELD OF THE INVENTION

This invention relates to a mixer and a method for mixing first andsecond fluids. The two fluids may be gases, e.g. air and a combustiblegas, or a gas and a liquid, e.g. air and a liquid fuel, or liquids. Themixer may, in particular, form part of a combustion device.

BACKGROUND OF THE INVENTION

A number of applications require that separate fluid streams bethoroughly mixed. One such application is catalytic combustion, wherethe fuel and air must be very well mixed prior to entry in the catalyst.This requirement also holds true for conventional lean-premix burners.However, current mixing techniques generally do not achieve homogeneousmixtures, and therefore the resulting combustion process is non-uniform;large temperature variations are observed, and significant NOx emissionsassociated with high-temperature areas are recorded.

Attaining high levels of mixedness between fluids is normallyaccompanied by large, undesirable pressure losses. The most promisingoption to date is that involving counter-swirling flows. However,complex aerodynamic designs (i.e. aerofoil sections) are essentialcomponents of such units, because the structures generating swirl mustnot form wakes including recirculation zones (which can lead toflashback and severe damage); furthermore, there is scope forsignificant improvement in mixing quality. For certain applications,e.g. those involving catalytic units, the catalyst inlet velocitydistribution must be as uniform as possible; this is a difficultobjective for conventional mixers which employ swirling flows. A furtherproblem with many units is that of flashback, i.e. the phenomenon whichentails a homogeneous flame moving upstream and into the mixer, oftenresulting in damage.

Venturi injectors are relatively simple devices for attaining reasonablemixing; however, the quality falls short of that achieved by theswirl-based concepts. Venturi units rely upon low local pressures todraw additive fluid into a carrier fluid; mixing is attained by virtueof the shear layer across the longitudinal jet of fluid, whose principalvelocity component is axial. U.S. Pat. No. 4,123,800 describes a mixerin which a certain degree of twisting motion is imparted to the flowdownstream of the Venturi constriction, to further aid in mixing, bymeans of skewed grooves machined into the walls of the divergent sectiondownstream of a throat section into which the additive fluid isinjected. However, this twisting motion is only imparted near the walls,without significantly affecting the bulk of the flow, and does notmeaningfully assist the mixing process.

SUMMARY OF THE INVENTION

The present invention provides a passageway along which a streamcomprising the first fluid flows along an axis of the passageway, thepassageway having, in sequence in the downstream direction, a convergentsection, a throat, and a divergent section; an injector for introducingthe second fluid into the stream in the passageway upstream of thedivergent section; and a swirl generator in the passageway upstream ofthe convergent section.

The invention also provides a method of mixing fluids, comprising thesequential steps of:

-   -   (a) providing a stream comprising a first fluid and having an        axis along which the stream flows,    -   (b) inducing swirl in the stream about its axis,    -   (c) causing the stream to converge towards its axis, and    -   (d) causing the stream to diverge from its axis,        The method including introducing a second fluid into the stream        before step (d).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by way of example only, withreference to the accompanying drawing, in which:

FIG. 1 is a schematic axial section through one embodiment of a mixer;

FIG. 2 is a graph of angular velocity, ω, in the circumferentialdirection against radial distance, r, from the axis of a swirling streamcreated in a preferred embodiment of the mixer;

FIG. 3 shows the angular velocity field produced by the swirling streamhaving the radial distribution of angular velocity shown in FIG. 2;

FIG. 4 a is a cross-section through the convergent section of a mixer,showing one possible arrangement of injectors;

FIG. 4 b in a view similar to FIG. 4 a, showing another possiblearrangement of injectors;

FIG. 5 is a schematic axial section through a mixer combined with aburner sector;

FIG. 6 is an inlet end view of the mixer in FIG. 5; and

FIG. 7 is an outlet end view of the burner sector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The mixer illustrated in FIG. 1 comprises a passageway 1 having an axis2 along which a stream of air (the carrier fluid or first fluid) flowsin the direction of the arrow 3. The passageway 1 has an upstream endportion or inlet section 4 which is cylindrical, a convergent section 6which is conical and which converges at an angle θ with respect to theaxis 2, a divergent section 7 which is conical and diverges at an anglea with respect to the axis 2, and a downstream end portion or outletsection 8 which is cylindrical. The passageway has a throat 9 betweenthe convergent and divergent sections 6, 7; in the embodimentillustrated, the throat 9 is of negligible axial length. The convergentsection 6, throat 9, and divergent section 7 together constitute aVenturi section.

An injector comprising a plurality of injection ports 11 in theperipheral wall 12 of the passageway 1 introduces fuel (the additivefluid or second fluid) into the stream in the convergent section 6 atmultiple locations along and around the axis 2. If the additive fluid isa liquid, it can be injected as sprays, and droplet atomisation andpenetration can be enhanced by using high-pressure injectors.

A swirl generator 13 is provided in the inlet section 4 of thepassageway 1. This imparts swirl to the bulk flow of the carrier fluidprior to the convergent section 6 and prior to the injection of thefuel. Conservation of angular momentum results in increased angularvelocities of the swirling stream at the throat 9. Such a configurationenhances mixing between the carrier fluid and additive fluid by virtueof the circumferential shear layers which are formed. These shear layerspromote cross-stream diffusion. Mixing begins earlier than in aconventional Venturi injector and results in a longer time beingavailable for mixing and a more uniform concentration profile.

Mixing can be significantly improved by generating angular velocityprofiles which form a number of intense circumferential shear layers.The intensity of these shear layers is directly proportional to dω/dr(the radial gradient of the angular velocity). Thus, abrupt changes inthe circumferential component of the angular velocity are desirable.FIG. 2 is a graph of the angular velocity, ω, in the circumferentialdirection against radial distance, r, from the axis 2, illustrating aradical form of such an angular velocity profile. A swirling velocityfield resulting from the application of inlet angular velocities similarto those of FIG. 2 is depicted in FIG. 3. Whilst distinct swirlingannular bodies are clearly visible at the inlet of the convergentsection 6, these are seen to merge with each other as the longitudinaldistance from the inlet increases. Such “blurring” of the annular layersis indicative of cross-stream interactions, which result in mixing ofthe first and second fluids.

In order to generate such an angular velocity profile, a swirl generator13 is used in which the swirl angle varies in the radial direction,typically increasing with distance from the axis.

Whilst strong levels of swirl are beneficial to mixing, vortex breakdownhas to be avoided if flashback is to be prevented in combustionapplications. Although studies have demonstrated that vortex breakdownis promoted by expansion downstream of a swirl generator, we have foundthat vortex breakdown does not occur so readily if a convergent sectionis placed between the swirl generator and the divergent section. Also,studies have shown that abrupt changes in tangential velocity profilestend to reduce the tendency of vortex breakdown. Instead, flashback ishindered by the strongly swirling axial jet which is formed. The valueof θ required to avoid vortex breakdown is a function of the angularvelocity profile produced by the swirl generator 13. For example, wehave found that (for a given operating condition, i.e. velocity,pressure, temperature) if the swirl angle varies (radially) between 15°and 45°, θ should lie between 15° and 25°, whereas in a configurationwhere the swirl angle changes from 15° to 30°, θ may be reduced to lessthan 15°.

The nature of the divergence downstream of the throat 9 can be selectedfor various needs. If recirculation zones are not desired, expansionmust not be sudden, so a more gradual increase in the cross-section ofthe divergent section 7 is needed. Such a configuration may beapplicable to cases where no negative axial velocities are desired, forexample in catalytic combustion. On the other hand, the mixer may beused for premixed combustion, in which case sudden expansion serves toaerodynamically anchor the homogeneous flame.

The mixer does not require the large inlet to throat diameter ratio(typically 2) normally necessary for strongly accelerating a carrierfluid, because of the high degree of mixing resulting from tangentialshear in the carrier fluid, for which the axial velocities need nolonger be so high. Conventional Venturi injectors require small anglesof divergent (diffuser angles), typically α=5°, if flow separation is tobe avoided, but the resulting long diffuser lengths result insignificant pressure losses such that, in a typical conventional Venturiinjector, 95% of the loss is incurred during diffusion. In the presentmixer, if flow separation is to be avoided, small angles of divergenceare still necessary, but the relatively large throat diameter results inshorter diffusers and hence smaller pressure losses. Such a saving ofspace may be highly advantageous in catalytic combustion applications.

The peripheral wall 12 of the passageway, particularly the Venturisection constituted by the convergent and divergent sections 6, 7, maybe coated with a catalytic material for the purpose of quenchingradicals, which are precursors of homogeneous ignition and combustion.This assists in preventing flashback and flame anchoring, these twophenomena being encouraged by the lower velocities encountered in theboundary layer near the peripheral wall.

The injection ports 11 may simply be holes which each face the axis 2.However, introducing the additive fluid in a direction which is skewedto the axis 2 results in increased turbulence and better mixing of theadditive fluid with the carrier fluid. FIGS. 4 a and 4 b show possibleorientations of the injection ports 11. In FIG. 4 a the ports 11 aresymmetrically arranged with respect to planes containing the axis of thepassageway. In FIG. 4 b the ports 11 are angled so as to assist theswirling motion of the carrier fluid. However, the injection ports mayinstead be angled in the opposite sense with respect to the swirldirection of the carrier fluid.

Injection ports 11 of different sizes may be provided in order toachieve different depths of penetration of the additive fluid into thestream. Fuels which are particularly prone to causing flashback due totheir high flames speeds and diffusivity, for examplehydrogen-containing gases such as synthesis gas, can be used in themixer because of the very high velocities achievable and the possibilityof avoiding recirculation zones. The swirl generator 13 may surround acentral member or mandrel, which may be in the form of a centralinjection tube for providing a central air jet hindering the formationof recirculating regions at the exit. Whilst the recirculation zoneswhich tend to form behind a solid mandrel would not normally cause flameattachment (because the axial velocities at the throat 9 are very high,typically tens of times the homogeneous flame speed of natural gas),extra caution has to be exercised when hydrogen-containing fuels areused because of the very high flame speed and diffusivity of hydrogen.

The swirl generator 13 may circumferentially surround a central fuelinjection lance, which could additionally inject air, in order tofurther enhance mixing.

Increasing the size of the mixer makes it possible to generate a largernumber of coaxial swirling layers. It may therefore be advantageous touse the mixer in a combustion device having multiple burners, by usingone mixer for a burner sector (comprising a number of burners). FIG. 5illustrates such an embodiment. The circular cross-section of thedivergent section 7 gradually changes into a sector of an annulus (FIG.7) in which a number of burners 14 are located (three burners beingshown by way of example). The burners 14 may be very simple (e.g.utilising sudden expansion without swirl) because complete fuel/airmixing has already been achieved prior to entry into the burners.

A flow straightener 16, which has also has the function of flashbackprevention is placed near the exit of the divergent section 7, upstreamof the burners 14. The flow straightener 16 has a similar constructionto the swirl generator 13, except that it has straight channels. Flowstraightening ensures that the flow distribution into each burner isidentical. Small channels (hydraulic diameter typically less than 5 mm)act as flame arrestors. The channels may be coated with a catalyst forquenching radicals, further hindering flashback. In order to minimisepressures losses, the flow straightener has a very small axial length,typically less than 15 mm. The fact that the mixing process is decoupledfrom the burners results in more uniform burner entry conditions (flowdistribution, pre-mixedness, temperature) and hence more uniformcombustion among the burners and fewer instabilities (which may arise ifthere are differences between burners). This embodiment is thusespecially applicable to catalytic combustion.

The embodiment of FIG. 5 can be used for liquid fuels if the geometryensures very high velocities such that the mixer residence time (i.e.the time taken for the fuel to move from the injection point to theburners) is very short, typically less than 3 ms at 3 bar.

Various modifications may be made within the scope of the invention. Inparticular, although the mixer has been particularly described in thecontext of mixing air and fuel, the mixer could be used for mixing anytwo (or more) different fluids. It is possible to introduce the secondfluid into the passageway at any convenient location upstream of thedivergent section 7. In particular, the throat 9 may be of substantiallength and the second fluid may be introduced into the throat.Alternatively, or additionally, the second fluid may be introduced intothe inlet section 4 (upstream or, preferably, downstream of the swirlgenerator 13). Alternatively, or additionally, the second fluid may beintroduced through a tube extending along the axis 2. It is alsopossible to introduce at least one further fluid into the passagewayupstream of the divergent section 7.

1. A mixer for mixing first and second fluids, comprising: a passagewayalong which a stream flows along an axis thereof, the stream comprisingthe first fluid, and the passageway comprising, in sequence in adownstream direction, a convergent section, a throat, and a divergentsection; an injector for introducing the second fluid into the stream inthe passageway upstream of the divergent section; and a swirl generatorin the passageway upstream of the convergent section.
 2. The mixer ofclaim 1, wherein the injector introduces the second fluid into thestream in the convergent section of the passageway.
 3. The mixer ofclaim 1, wherein the injector comprises at least one injection port in aperipheral wall of the passageway.
 4. The mixer of claim 3, wherein theinjector comprises at least two injection ports of different sizes. 5.The mixer of claim 1, wherein the injector introduces the second fluidat a plurality of locations along the passageway.
 6. The mixer of claim1, wherein the injector introduces the second fluid in at least onedirection which is transverse to the axis of the passageway.
 7. Themixer of claim 1, wherein the swirl generator has a swirl angle thatvaries as a function of distance from the axis.
 8. The mixer of claim 7,wherein the swirl angle varies such that there is at least one abruptchange in circumferential velocity of the stream about the axis, betweenone radial position and another.
 9. The mixer of claim 1, furthercomprising a central injection tube opening in the passageway upstreamof the divergent section.
 10. The mixer of claim 9, wherein the swirlgenerator circumferentially surrounds the central injection tube. 11.The mixer of claim 1, wherein upstream and downstream ends of theconvergent section have a diameter ratio of less than
 2. 12. The mixerof claim 1, wherein the convergent section converges at an angle of atmost 25° with respect to the axis of the passageway.
 13. The mixer ofclaim 12, wherein the angle is at least 10°.
 14. The mixer of claim 12,wherein the angle is at least 15°.
 15. The mixer of claim 1, wherein aperipheral wall of the passageway comprises a coating of a catalyst forquenching radicals that are precursors to ignition of a mixture of thefirst and second fluids.
 16. The mixer of claim 1, further comprising aflow straightener in the passageway downstream of the throat.
 17. Themixer of claim 16, wherein the flow straightener comprises channels witha hydraulic diameter of at most 5 mm.
 18. The mixer of claim 16, whereinthe flow straightener carries a catalyst for quenching radicals.
 19. Themixer of claim 16, wherein the flow straightener has a length in theaxial direction of at most 15 mm.
 20. The mixer of claim 1, wherein thepassageway forms part of a combustion device.
 21. The mixer of claim 20,wherein the combustion device comprises a plurality of burners suppliedwith a mixture of the fluids by the passageway.
 22. The mixer of claim21, wherein the burners form a burner sector, a downstream end portionof the passageway gradually changing cross-section into a sector of anannulus.
 23. A method of mixing fluids, comprising the sequential stepsof: (a) providing a stream comprising a first fluid and having an axisalong which the stream flows; (b) inducing swirl in the stream about anaxis thereof; (c) causing the stream to converge towards the axis; and(d) causing the stream to diverge from the axis; wherein a second fluidis introduced into the stream before step (d).
 24. The method of claim23, wherein the second fluid is introduced into the stream in adirection that is transverse to the axis of the stream.
 25. The methodof claim 23, wherein the second fluid is introduced into the streamduring step (c).
 26. The method of claim 25, wherein the second fluid isintroduced into the stream in a direction that is transverse to the axisof the stream.
 27. The method of claim 23, wherein a swirl angle of theswirl induced in step (b) varies as a function of distance from theaxis.
 28. The method of claims 23, wherein the swirl induced in step (b)is such that there is at least one abrupt change in circumferentialvelocity of the stream about the axis, between one radial position andanother.
 29. The method of claim 23, further comprising introducing afluid into a central region of the stream after step (b) and before step(d).
 30. The method of claim 29, wherein the fluid is the first fluid.31. The method of claim 29, wherein the fluid is the second fluid. 32.The method of claim 23, wherein the first and second fluids are bothintroduced into a central region of the stream.
 33. The method of claim23, further comprising straightening the flow of the stream after step(d).
 34. The method of claim 23, wherein at least one of the first andsecond fluids is a gas.
 35. The method of claim 34, wherein the firstfluid is air and the second fluid is a combustible fluid.
 36. The methodof claim 34, wherein the first fluid is a gas and the second fluid is aliquid.
 37. The method of claim 36, in which the first fluid is air andthe second fluid is a combustible fluid.